Biaxially oriented polylactic acid-based resin films

ABSTRACT

The present invention provides a biaxially oriented film of a polylactic acid resin that is suitable for films for bags and packaging films for various windows, particularly films for outlook window envelopes. The film is biodegradable and, as films for outlook window envelopes, is superior in the coating adaptability for an antistatic agent, a lubricant and an antiblocking agent or the like and in the high-speed cutting property. The present invention discloses a biaxially oriented film of a polylactic acid resin comprising not less than 50% by weight of a polylactic acid resin, which has a storage modulus E′at 80° C. within the range from 10 MPa to 3,000 MPa, as determined by the test method for temperature dependency of dynamic viscoelasticity, in accordance with JIS K7198 (method A); a heat shrinkage of not higher than 10%, upon heating at 80° C. for 10 seconds; and a tear strength in the width direction (TD direction) of the film of 10 to 200 mN, as determined in accordance with JIS K7128 (method B).

TECHNICAL FIELD

The present invention relates to a biodegradable biaxially oriented filmconsisting mainly of a polylactic acid resin. More specifically, thepresent invention relates to a biodegradable film for bags, packagingand for various windows with superior coating adaptability andhigh-speed cutting property.

BACKGROUND ART

Synthetic polymer compounds have been widely used as plastics due totheir superior characteristics. With the increase in the consumption ofthe synthetic polymer compounds, however, the amount of waste has beenalso increasing. It has thus become a social problem how the wasteplastics should be dealt with. Incineration causes problems, such as thedamage to furnaces due to high heat generation and the risk of emissionof toxic substances. Landfill also cause a problem that the plasticsstay in the environment for good since they do not decay. Furthermore,considering the costs of classification, collection and regeneration, itis difficult to completely solve the problem only by recycling.

In the midst of such increasing environmental concern, needs forbiodegradable plastics which decompose in the natural environment afterbeing wasted have been increasing to reduce environmental load and torealize the sustainable society.

Known biodegradable plastics include starch-based ones,aliphatic-polyester resins produced by microorganisms, chemicallysynthetic aliphatic polyester resins and the same resins but partiallymodified in their chemical structure, and biodegradable aliphaticaromatic polyester resins.

Among these biodegradable plastics, polylactic acid resins have superiortransparency, rigidity and processability, as compared with otherbiodegradable plastics. In particular, oriented films of polylactic acidresins are suitable as various films for packaging such as bags, windowfilms for containers with window, plastering films for outlook windowenvelopes, substitute films for cellophane, and the like, due to theirhigh stiffness and high transparency.

On the other hand, in the field of plastic film applications, processingspeed has been increasing with the progress of processing machinery. Forexample, in the affixing of films for envelope windows, recentprocessing speed is 800 to 1,000 sheets/minute, or even 1000 or moresheets/minute due to the progress of window affixing machine, while theconventional processing speed was 400 to 600 sheets/minute. For thisreason, running speed of the film also has been increasing and thusthere are needs for a film with superior high-speed cutting property. Inaddition, there are applications in which film surface is to be coatedwith an antistatic agent, a lubricant, an antiblocking agent, or thelike, to provide the film with machine adaptability and processabilityto suit the high-speed processing. For example, the above-describedcoating is indispensable in bags, packaging films and films for variouswindows, particularly those for window of outlook window envelopes intowhich relatively light weight materials easily affected by staticelectricity, such as powder, granule, thin papers, film, fiber-likesubstance, are contained. Therefore, the films are required to havecoating adaptability, in addition to the high-speed cutting property.Biodegradable films, in particular polylactic acid film, with superiortransparency and mechanical properties, satisfying these requirementshave not been obtained.

JP-A-2001-122989 discloses that a biaxially oriented polylactic acidpolymer film consisting of crystalline polylactic acid and having thestorage modulus E′ at 120° C. within the range from 100 MPa to 230 MPa,as determined by the test method for temperature dependency of dynamicviscoelasticity, is suitable for fold-packaging due to superior tackingproperty. This film, however, cannot satisfy the coating adaptabilityand high-speed cutting property. JP-A-2000-198913 discloses an easilytearable, biaxially oriented film of a polylactic acid resin consistingof polylactic acid and crystalline aliphatic polyester. This film,however, only has limited applications due to significant haze.JP-A-2001-64413 discloses an easily tearable, biaxially oriented film ofa polylactic acid resin consisting of polylactic acid and polyethyleneterephthalate and/or polyethylene isophthalate. This film, however, onlyhas limited applications due to incomplete biodegradability andinsufficient coating adaptability and high-speed cutting property,although it is superior in straight tearing and hand tearing properties.JP-A-2001-354789 also discloses biodegradable polylactic acid resinfilms with a good balance among antistatic property, lubricity andantiblocking property, along with superior adhesion to paper. This film,however, does not have the high-speed cutting property.

The object of the present invention is to provide a biodegradable,biaxially oriented film of a polylactic acid resin suitable for filmsfor bags, packaging and films for various windows with superior coatingadaptability for antistatic agents, lubricants, antiblocking agents orthe like, as well as high-speed cutting property.

DISCLOSURE OF THE INVENTION

The present inventors have found, after extensive studies to solve theabove-described problems, that a biodegradable, biaxially oriented filmof a polylactic acid resin, which is suitable for films for bags,packaging and films for various windows with superior coatingadaptability as well as high-speed cutting property, can be obtained bymaking the storage modulus E′ at 80° C., as determined by the testmethod for temperature dependency of dynamic viscoelasticity, inaccordance with JIS K7198 (method A), the heat shrinkage at 80° C. andthe tear strength in the width direction (TD direction) of a film withinthe specific ranges, respectively, despite the fact that the physicalproperties of the film are complicatedly influenced by its blendcomposition of crystalline polylactic acid and amorphous polylacticacid, drawing temperatures and drawing ratios in the MD and TDdirections, and heat treatment conditions and the like. Based on thesefindings, the present invention has been accomplished.

Namely, the present invention comprises the following aspects:

(1) A biaxially oriented film of a polylactic acid resin comprising notless than 50% by weight of a polylactic acid resin,

-   -   which has a storage modulus E′ at 80° C., in at least one of a        longitudinal direction (MD direction) and a width direction (TD        direction) of the film, of from 10 MPa to 3,000 MPa, as        determined by the test method for temperature dependency of        dynamic viscoelasticity in accordance with JIS K7198 (method A);    -   a heat shrinkage of not higher than 10%, upon heating at 80° C.        for 10 seconds; and    -   a tear strength in the width direction (TD direction) of 10 to        200 mN, as determined in accordance with JIS K7128 (method B).

(2) The biaxially oriented film of a polylactic acid resin in accordancewith (1), wherein the storage modulus E′ at 80° C., in at least one ofthe longitudinal direction (MD direction) and the width direction (TDdirection) of the film, is within the range from 50 MPa to 1,000 MPa, asdetermined by the test method for temperature dependency of dynamicviscoelasticity.

(3) The biaxially oriented film of a polylactic acid resin in accordancewith (1) or (2), wherein the storage modulus E′ at 80° C., in at leastone of the londitudinal direction (MD direction) and the width direction(TD direction) of the film, is within the range from 10 MPa to 300 MPa,as determined by the test method for temperature dependency of dynamicviscoelasticity; and a heat of fusion, ΔHm, at the crystal melting peak,present at a temperature not lower than 100° C., is in the range from 15to 30 J/g, as determined by a differential scanning calorimeter (DSC)with a temperature being increased from 0° C. to 200° C. in accordancewith JIS K7122.

(4) The biaxially oriented film of a polylactic acid resin in accordancewith (1) or (2), wherein the polylactic acid resin is a mixturecomprising of 95 to 60 parts by weight of crystalline polylactic acidwith an optical purity of not lower than 85% and 5 to 40 parts by weightof amorphous polylactic acid with an optical purity of not higher than80%.

(5) A window for an outlook window envelope, comprising the biaxiallyoriented film of a polylactic acid resin in accordance with (1) or (2).

(6) The biaxially oriented film of a polylactic acid resin in accordancewith (1) or (2), wherein said film is drawn at a ratio of not less than4 in the width direction (TD direction) of the film, and is subjected toa heat treatment at a temperature not lower than the glass transitiontemperature (Tg) thereof and not higher than the melting point (Tm)thereof.

(7) A method for producing a biaxially oriented film of a polylacticacid resin, comprising:

-   -   drawing a film comprising a resin containing not less than 50%        by weight of a mixture comprising 95 to 60 parts by weight of        crystalline polylactic acid with an optical purity of not lower        than 85% and 5 to 40 parts by weight of amorphous polylactic        acid with an optical purity of not higher than 80%, at a ratio        of not less than 4 in the width direction (TD direction) of the        film; and    -   subsequently subjecting the film to a heat treatment at a        temperature not lower than the glass transition temperature (Tg)        thereof and not higher than the melting point (Tm) thereof.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be explained in more detail hereinbelow.

A film of the present invention comprises not less than 50% by weight,preferably not less than 70% by weight, more preferably not less than85% by weight, and further more preferably 100% by weight of, apolylactic acid resin. If the content of a polylactic acid resin is lessthan 50% by weight, the elastic modulus and transparency of PLA tend tobe lowered. Said polylactic acid resin means: a copolymer of polylacticacid or lactic acid and other hydroxycarboxylic acid(s), aliphaticcyclic ester(s), dicarboxylic acid(s) and/or diol(s), containing notless than 85% by weight of lactic acid monomer units; or a compositionof these polymers containing not less than 85% by weight of lactic acidmonomer units.

Lactic acid has L-lactic acid and D-lactic acid as optical isomers, andpolylactic acid obtained by polymerization of lactic acid includescrystalline polylactic acid and amorphous polylactic acid. The formercomprises about 10% or less of D-lactic acid units and about 90% or moreof L-lactic acid units, or about 10% or more of L-lactic acid units andabout 90% or more of D-lactic acid unit, and has an optical purity ofnot lower than about 80%, while the latter comprises 10% to 90% ofD-lactic acid units and 90% to 10% of L-lactic acid units, and has anoptical purity of not higher than about 80%.

A polylactic acid resin used in the present invention is preferably apolylactic acid resin that is a mixture comprising 100 to 60 parts byweight of crystalline polylactic acid having an optical purity of notlower than 85% and 0 to 40 parts by weight of amorphous polylactic acidhaving an optical purity of not higher than 80%. Particularlypreferably, a polylactic acid resin is a mixture of 95 to 60 parts byweight of crystalline polylactic acid having an optical purity of notlower than 85% and 5 to 40 parts by weight of amorphous polylactic acidhaving an optical purity of not higher than 80%. The resins having thecompositions in these ranges are advantageous as an oriented film withsuperior heat sealing property while having the heat shrinkagesuppressed to a low level after heat treatment, can be easily obtainedtherefrom. If the content of crystalline polylactic acid having anoptical purity of not lower than 85% is less than 60 parts by weight,heat shrinkage tends to increase.

The hydroxycarboxylic acid(s), a monomer(s) to be used as acopolymerization component with lactic acid, include such as glycolicacid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 3-hydroxyvalericacid, 4-hydroxyvaleric acid and 6-hydroxycaproic acid.

The aliphatic cyclic ester(s) includes glycolide, lactide,β-propiolactone, γ-butyrolactone, δ-valerolactone, ε-caprolactone andsubstituted lactones with various groups such as a methyl group, and thelike. The dicarboxylic acid(s) includes succinic acid, glutanic acid,adipic acid, azelaic acid, sebacic acid, terephthalic acid andisophthalic acid, and the like. The polyvalent alcohol(s) includesaromatic polyvalent alcohols, such as bisphenol/ethylene oxide adducts;aliphatic polyvalent alcohols, such as ethylene glycol, propyleneglycol, butanediol, hexanediol, octanediol, glycerin, solbitan,trimethylolpropane, neopentyl glycol; and ether glycols, such asdiethylene glycol, triethylene glycol, polyethylene glycol,polypropylene glycol.

Biodegradable plastics, such as aliphatic polyesters, may be added tothe above-described polylactic acid. The amount to be added, however,should be not higher than 50% by weight so as not to impair theproperties of the polylactic acid resin itself.

As a method of polymerizing the polylactic acid resin, a known method,such as a polycondensation method, a ring opening polymerization methodand the like can be employed. A method for increasing molecular weightby using a binder, such as polyisocyanate, polyepoxy compound, acidanhydride and polyfunctional acid chloride, can also be employed. Theweight average molecular weight of the polylactic acid resin ispreferably in the range from 20,000 to 1,000,000, more preferably in therange from 30,000 to 500,000. If the molecular weight is within theseranges, the melt viscosity will be in a suitable range and, thus, astable film can be obtained even by using usual processing machine.

The biaxially oriented film of a polylactic acid resin of the presentinvention has a storage modulus E′ at 80° C. of from 10 MPa to 3,000 MPaat least in one of the MD and TD directions, as determined by the testmethod for temperature dependency of dynamic viscoelasticity, inaccordance with JIS K7198 (method A). It is preferable that the storagemodulus is in the above range at least in the MD direction, morepreferably in both of the MD and TD directions. Since films are heatedup to around 80° C. by hot air for drying in a drying step after coatingwith an antistatic agent, a lubricant, an antiblocking agent or thelike, if the storage modulus E′ at 80° C. of a film is less than 10 MPa,the film will have deformations, such as film elongation by tensionexerted on film in drying process, which makes it difficult to obtain agood state of roll of film. On the other hand, a film of polylactic acidresin with a storage modulus E′ thereof at 80° C. of higher than 3,000MPa is practically difficult to obtain. Preferable range of E′ of a filmat 80° C. is from 50 MPa to 1,000 MPa, and particularly preferable rangeis from 70 MPa to 500 MPa. A film with E′ at 80° C. of not higher than300 MPa is preferable as it has the superior heat sealing property.

The biaxially oriented film of a polylactic acid resin of the presentinvention has heat shrinkage, in both the MD and TD directions, of nothigher than 10%, upon heating at 80° C. for 10 seconds. The film withheat shrinkage over 10%, upon heating at 80° C. for 10 seconds, exhibitsheat shrinkage by hot air for drying in drying process after coatingwith an antistatic agent, a lubricant, an antiblocking agent or thelike, to have wrinkles on the film surface. It is therefore difficult toobtain a film with good appearance. The heat shrinkage upon heating at80° C. for 10 seconds is preferably not higher than 5%, and particularlypreferably not higher than 3%.

Furthermore, the biaxially oriented film of a polylactic acid resin ofthe present invention is required to have a tear strength in the widthdirection of the film (TD direction) of 10 to 200 mN, as determined inaccordance with JIS K7128 (method B). A film with the tear strength inthe TD direction over 200 mN has inferior high-speed cutting propertyand, thus, is not adaptable to high-speed bag manufacturing equipmentand high-speed fabrication machine, in particular to high-speed affixingmachine for envelope windows. On the other hand, a film with said tearstrength lower than 10 mN has undesired film breaks at high frequencyduring the slitting work. Preferable range of said tear strength is from20 to 150 mN, and particularly preferable range is from 20 to 100 mN.

Furthermore, the biaxially oriented film of a polylactic acid resin ofthe present invention preferably has a heat of fusion, ΔHm, at thecrystal melting peak present at a temperature not lower than 100° C., of15 to 30 J/g, in particular, 22 to 28 J/g, as determined by adifferential scanning calorimeter (DSC) with the temperature beingincreased from 0° C. to 200° C. in accordance with JIS K7122. A filmwith the above value not higher than 30 J/g is superior in heat sealingproperty, while a film with the value not smaller than 15 J/g exhibitslittle heat shrinkage and, thus, hardly shrinks during hot air dryingafter coating.

The biodegradable film of the present invention has a haze of preferablynot higher than 5%, more preferably not higher than 3%, and particularlypreferably not higher than 2%, as measured by a hazemeter (ASTM D1003-95). If the haze is not lower than 5%, the transparency will bereduced to make it difficult to see contents in the bag. In the case ofgeneral packaging film, it is difficult to see contents clearly throughthe film, and thus impairs the appearance and value as a commercialproduct. In the application of the outlook window envelopes, inparticular, if the haze is not lower than 5%, the transparency will bereduced to frequently cause reading error of the information recorded ina bar code.

The polylactic acid resin film of the present invention may becompounded with, in addition to the above-described resins, knownadditives such as other biodegradable resins, heat stabilizers,antioxidants and UV absorbers, in an amount not to impair therequirements in and the characteristics of, the present invention. Inthe case of mixing a resin which increases the haze, the maximum amountto be compounded should be 5% based on the polylactic acid resin.

The thickness of a film of the present invention is preferably in therange from 5 to 100 μm, more preferably in the range from 7 to 50 μm. Inparticular, the thickness of film for envelope windows is preferably inthe range from 20 to 40 μm to provide stiffness suitable for affixingmachine for the outlook window envelopes.

Hereinbelow, a method for producing a biaxially oriented film comprisingnot lower than 50% by weight of a polylactic acid resin of the presentinvention will be described. First, a raw material resin(s) is fed intoa single screw or twin screw extruder and melt mixed. The melt is thenextruded through a T die, a cylinder die or an I die and quenched to bein an amorphous-like state. The melt extrusion temperature range at thistime is, in general, preferably from 100 to 250° C. The quenchingtemperature is preferably from 0 to 60° C. The resin is then biaxiallydrawn by a conventionally known drawing method including a roll method,a tenter method, a tubular method or the like. Among these drawingmethods, the tenter method is preferable due to easiness to obtain aoriented film of the present invention because drawing ratios in the MDand TD directions can be independently controlled.

Drawing ratio in the drawing procedure may be selected from the range of1.5 to 10 each for the longitudinal direction (MD direction) and thewidth direction (TD direction) of the film. The drawing ratio ispreferably at least 4 in the TD direction, in view of the enhancement offilm strength and film cutting property in the TD direction by impartingorientation, more preferably at least 2 in the MD direction and at least4 in the TD direction, particularly preferably from 2.0 to 4.5 in the MDdirection and from 4.5 to 7 in the TD direction. The ratios are furtherparticularly preferably from 2.5 to 3.5 in the MD direction and from 5.2to 7 in the TD direction. The ratio of (drawing ratio in the TDdirection)/(drawing ratio in the MD direction) is preferably larger than2.0. By drawing the film in the TD direction at a ratio of 4 or more,the tear strength in the TD direction falls within a specified range andthe high-speed film cutting property is improved. Preferable drawingtemperature is from 65° C. to 90° C.

To obtain the biaxially oriented film of a polylactic acid resin of thepresent invention, the film is heat treated at a temperature range notlower than the glass transition temperature and not higher than themelting point of the polylactic acid resin used as a raw material, afterthe biaxial drawing. When a tubular method is employed, a heat treatmentmethod wherein a film is heated by external hot air and the like, whilebeing kept in tensed state by maintaining the internal pressure bysealing internal air, is usually employed. In the tenter method, a heattreatment method wherein a film is passed through a heat treatment zonewith the film being hold with clips. Preferably, the film is heattreated at a temperature of not lower than the drawing temperature butnot higher than the melting point of a film for a period not shorterthan 1 second depending on the treatment temperature, and particularlypreferably heat-treated at a temperature range not lower than 100° C.and not higher than the melting point for a period not shorter than 2seconds. If the temperature is too low or if the heat treatment time istoo short, the heat shrinkage of the drawn film may exceed 10%, uponheating at 80° C. for 10 seconds. Heat treatment in the state where thetension in the TD direction and/or the MD direction is relaxed iseffective to reduce the heat shrinkage. Heat treatment under excessiverelaxation, however, tends to increase tear strength in the TD directionresulting in lower high-speed cutting property. Thus, it is preferableto carry out the heat treatment in the state where the tension to thefilm is relaxed to such an extent that the tear strength in the TDdirection becomes not higher than 200 mN, in order to obtain a film withsuperior high-speed cutting property.

The film of a polylactic acid resin is more hydrophilic than films basedon a polyolefin resin or a polystyrene resin. However, it is preferableto impart further hydrophilicity to the film by a method such as coronatreatment of the film surface to be coated, for uniform coating of thebiaxially oriented film of a polylactic acid resin of the presentinvention with an antistatic agent, a lubricant, an antiblocking agentor the like. This hydrophilicity imparting treatment improves theuniformity of the coating and effectively provides antistatic propertyor lubrication effect. Said hydrophilicity imparting treatment ispreferably carried out so that the surface tension becomes in the rangefrom 400 μN/cm to 600 μN/cm.

The biaxially oriented film of a polylactic acid resin of the presentinvention is biodegradable and useful as films for bags, packaging andvarious containers with windows, to which an antistatic agent, alubricant, an antiblocking agent or the like is applied, since it hassuperior coating adaptability and high-speed cutting property. It isespecially suitable for films for the outlook window envelopes. Thefilm, when used for bags or packaging, preferably has a tensilestrength, as mechanical strength, of not lower than 20 MPa and a tensileelongation of from 20 to 1,000%. These values are specified depending onthe applications.

EXAMPLES

The present invention will now be explained by way of Examples andComparative Examples.

First, evaluation methods used in the Examples and Comparative Examplesare described below.

(1) Composition of D- and L-Lactic Acids in Polylactic Acid Polymer andOptical Purity

The optical purity of polylactic acid polymer is calculated by thefollowing equation based on a composition ratio of L-lactic acid unitsand/or D-lactic acid units composing the polylactic acid, as describedabove.

-   -   optical purity(%)=|[L]−[D]|    -   wherein [L]+[D]=100, and |[L]−[D]|represents an absolute value        of [L]−[D].

Composition ratio of L-lactic acid units and/or D-lactic acid unitscomposing polylactic acid polymer is determined by:

-   -   preparing a hydrolyzed sample (liquid) by alkaline decomposition        of a sample with 1N-NaOH, followed by neutralizing with 1N—HCl        and adjusting the concentration with distilled water;    -   passing the hydrolyzed sample through a high performance liquid        chromatography (HPLC: LC-10A-VP) from Shimadzu Corp. to obtain        an area ratio of detected peaks (area is measured by a vertical        line method) corresponding to L-lactic acid and D-lactic acid at        254 nm UV;    -   obtaining a weight ratio of L-lactic acid [L] (unit: %)        composing the polylactic acid polymer and a weight ratio of        D-lactic acid [D] (unit: %) composing the polylactic acid        polymer from the area ratio; and    -   taking the mean (rounded) of three measurement values per        polymer as the measurement value of the composition ratio.        (2) Weight Average Molecular Weight Mw of Polylactic Acid        Polymer

The weight average molecular weight Mw was determined using a gelpermeation chromatography equipment (GPC: data processing part;GPC-8020, detection part; RI-8020) from Toso Co., Ltd., under thefollowing measuring conditions, as polystyrene equivalent value based onthe standard polystyrene. Three measurement values per polymer werearithmetically averaged and rounded and the average was employed as themeasurement value.

Column: connected column of Shodex K-805 and K-801 from Showa Denko Co.,Ltd. (7.8 mm diameter×60 cm length)

-   -   Eluate: chloroform    -   Concentration of sample solution: 0.2 wt/vol %    -   Volume of sample solution charged: 200 μL    -   Flow rate of solvent: 1 ml/min.    -   Column/detector temperature: 40° C.        (3) Glass Transition Temperature (Tg), Melting Point (Tm) and        Crystal Heat of Fusion (ΔHm)

A sample was heated from 0° C. to 200° C. in differential scanningcalorimeter (DSC) in accordance with JIS-K7121 and JIS-K-7122 to measureTg, Tm and heat of fusion ΔHm of crystal melting peak, present at atemperature not lower than 100° C. That is, about 10 mg of test samplewas cut out from a sample film conditioned (by leaving to stand for 1week) in the standard state (23° C., 65% RH) and the DSC curve of thesample was drawn with a differential scanning calorimeter model DSC-7(heat flux type DSC) from Perkin-Elmer Inc., with nitrogen gas beingflowed at a rate of 25 ml/min and temperature being increased from 0° C.to 200° C. at a rate of 10° C./min. Melting point Tm (° C.) wasdetermined from a top of melting (endothermic) peak in the temperatureincreasing process, crystal heat of fusion ΔHm(J/g) from an endothermicpeak area, and Tg (° C.) from a crossing point (midpoint glasstransition temperature) of step-wise changing part of a curve in thetemperature increasing process and a line with equal distance invertical axis direction, from extended lines of both baselines. Fourmeasurement values per polymer were arithmetically averaged and roundedand the average was employed as the measurement value.

(4) Drawing Ratios in the MD Direction and TD Direction

Drawing ratios in the MD direction and TD direction were determined bythe following equations.

Drawing ratio in the MD direction=(film flow rate after drawing in theMD direction)/(original sheet flow rate before drawing in the MDdirection)

Drawing ratio in the TD direction=(film width after drawing in the TDdirection)/(original sheet width before drawing in the TD direction)

In a tenter method, film or sheet width means a width between clipsbefore and after drawing in the TD direction.

(5) Storage Modulus E′ (MPa)

Storage modulus E′ was measured in accordance with JIS K7198 (method A).Namely, it was measured by a tensile vibration method under theconditions of frequency of 1 Hz and temperature increasing from 20° C.to 160° C. at a rate of 2° C./min, with a test piece having a width of 7mm being held so that the distance between the chucks was 22 mm, usingRSA-II from Rheometric Scientific F.E. Inc.

(6) Heat Shrinkage (%)

A test piece with 150 mm×150 mm was cut out from a film sample in such amanner that one film side was parallel to the MD direction. On the testpiece a square of 100 mm×100 mm was drawn in such a manner that one sidethereof was parallel to the MD direction. In the square, two sets ofnine straight lines were drawn at 10 mm intervals with the sets being inparallel to the MD direction and the TD direction, respectively, toprepare the test piece on which the squares of 10 mm×10 mm were drawn.The test piece was placed in a hot air drying chamber set at 80° C. for10 seconds and allowed to shrink freely. Heat shrinkage was determinedby taking the mean of the values from the following equation and thedimensions of 11 lines drawn in the MD direction and TD direction.

Heat shrinkage (%)=[(line dimension before heat shrinkage)−(linedimension after heat shrinkage)]/(line dimension before heatshrinkage)×100

(7) Haze (%)

A test piece of square film with a size of 50 mm×50 mm and a thicknessof 25 μn was cut out from a film sample conditioned (by leaving to standfor 1 week at 23° C.) under the standard conditions (23° C., 65% RH)Haze (%) was measured at the standard condition in accordance with ASTMD1003-95, using a hazemeter, model NDH-1001DP from Nippon Denshoku Ind.Co., Ltd. Six measurement values per film type were arithmeticallyaveraged and rounded to obtain the haze.

(8) Tear Strength (mN)

Tear strengths (mN) in the MD direction and TD direction of a film wasmeasured in accordance with JIS K7128 (method B).

(9) Coating Adaptability

Coating adaptability was evaluated by using a coating compositionimproving antistatic property and lubricity (consisting of 50% by weightof “TSF-4441” from Toshiba Silicone Co., Ltd. as a polyether-modifiedsilicone and 50% by weight of “Nymeen F-215”,polyoxyethylene-alkyl(coconut oil)amine from NOF Corp. as a surfactant)as a surface treatment agent. First, the film surface to be coated wassubjected to corona treatment so as to impart a surface tension of 500μN/cm, using model AGI-060MD from Kasuga Electric Machine Co., Ltd. Thefilm surface was then coated with an aqueous solution of said coatingcomposition having a concentration of the surface treatment agent of0.3% by weight, using a spray coater. The film was passed through a hotair dryer set at 90° C. to remove moisture. The coating amount wascontrolled to be 2.5 mg/m² by adjusting the conditions of the spraycoater (air pressure and line speed). Coating adaptability was evaluatedaccording to the following criteria from the film coating state and theheat shrink state after the coating and hot air drying.

∘: a film having a good coating, showing uniform coating withoutgeneration of wrinkle, film extension, sagging, or the like caused byheat shrink.

⊚: a film having a coating practically usable without problem, showinguniform coating and a little generation of wrinkles, film extension,sagging, or the like caused by heat shrink.

X: a film not suitable for practical use, showing generation of muchwrinkles, film extension, sagging, or the like caused by heat shrink.

(10) High-Speed Cutting Property and Envelope Window AffixingAdaptability

To evaluate high-speed film cutting property and envelope windowaffixing adaptability, the upper limit of practically possible cuttingspeed at which window can be affixed without displacement was measuredby affixing test for envelope window on a window frame of 50×90 mm on anenvelope of 135×235 mm (made of paper), under various cutting speeds,using envelope window affixing machine (model HELIOS 202 fromWINKLER+DUNNERBIER), followed by visual inspection of displacement withrespect to the a envelope window frame. High-speed cutting property andenvelope window affixing adaptability were evaluated according to thefollowing criteria based on these test results.

⊚: a film that can be cut at a cutting speed of 600 sheets or more/minand affixed without causing displacement

∘-⊚: a film that can be cut at a cutting speed of 500 to 600 sheets/minand affixed without causing displacement

∘: a film that can be cut at a cutting speed of 400 to 500 sheets/minand affixed without causing displacement

X: a film that cannot be cut at a cutting speed of not lower than 400sheets/min or a film that causes displacement due to delay, even ifcutting at such a speed is possible

(11) Tensile Strength (MPa)

Tensile strengths (MPa) of the film in the MD direction and TD directionwere measured in accordance with ASTM D882.

(12) Tensile Elongation (%)

Tensile elongations (%) of the film in the MD direction and TD directionwere measured in accordance with ASTM D882.

(13) Heat Sealing Property

Three test pieces of rectangular film with a length of 250 mm in thelongitudinal direction (the MD direction)×a width of 25.4 mm (1 inch)were cut out from a film sample conditioned (by leaving to stand for 1week at 23° C.) under the standard condition (23° C., 65% RH). Hot tackstrength (HT strength; unit: N/1 inch width) was measured in accordancewith ASTM-F1921-98, as a peak strength observed within 1,000 mS (=1 sec)after die opening, using a hot tack measurement instrument from ThellerLtd. under the following sealing conditions.

Shape of the upper die: metal die, V type of 60 degree (tipcross-section of half-circle shape with R=1 mm, 5.25 inch length)

Shape of the lower die: die with rubber lining, plain type (0.5 inchwidth×5.25 inch length)

Dimension of seal surface: 1 inch×1 mm

-   -   seal temperature: (upper die) 110° C., (lower die) 25° C.    -   seal time: 1,000 mS    -   seal pressure: 13±1 MPa

Heat seal property of a film was evaluated by hot tack strength (HTstrength: peak strength, unit: N/1 inch width), corresponding tohigh-speed heat seal strength in packaging machine or bag makingmachine, in view of continuous heat seal stability not to cause breakingout of the content from the seal part or partial peeling off (orbreakage) of seal part, when it is continuously subjected to heatsealing process from original wound film state to form packaging or bagsby packaging machine or bag making machine, according to the followingcriteria.

∘: a film having a hot tack strength of not lower than 7N/1 inch widthand sufficient strength and no breaking out of its content or seal linefailure, showing a very good state

⊚: a film having a hot tack strength of not lower than 5N/1 inch widthwhich is at the level where the film can be practically used withoutproblem, and no breaking out of its content or seal line failure

X: a film having a hot tack strength of lower than 5N/1 inch widthwherein seal line may be peeled off (broken) and the content may bebroken out

Polylactic acid resins used in the following Examples 1 to 16 andComparative Examples 1 to 6 were crystalline polylactic acids (a), (b)and amorphous polylactic acid (c) having the weight average molecularweights and the optical purities shown in Table 1, obtained by thepolymerization with the catalyst amount, polymerization conditions,monomer composition and the like being controlled in accordance withExamples 1B to 7B in JP-A-4-504731. However, the compositions of thepolylactic acid resins in the present invention should not be limitedthereto. TABLE 1 Weight Content of average D-lactic Optical Mw acidpurity Tg Tm Crystalline 250,000 4.5% 91% 57° C. 153° C. polylactic acid(a) Crystalline 270,000 1.5% 97% 58° C. 170° C. polylactic acid (b)Amorphous 280,000 13.0% 74% 53° C. None polylactic acid (c)

Examples 1 to 11 and Comparative Examples 1 to 6

In Examples 1 to 11 and Comparative Examples 1 to 6, pellets ofcrystalline polylactic acid (a) or (b) and amorphous polylactic acid (c)shown in Table 1 were dry blended to the compositions shown in Table 2,followed by melt blending using a co-rotating twin screw extruder,extruding molten resins at a resin temperature of 200° C. through aT-die, quenching by a casting roll kept at a temperature of 35° C. toobtain substantially amorphous sheets. Then the sheets obtained wereheated at 75° C., roll-drawn in the MD direction at the drawing ratiosshown in Table 2, and then tenter-drawn in the TD direction at 85° C. atthe drawing ratios shown in Table 2. Thereafter, all the films, exceptfor the film in Comparative Example 2, were introduced into a heattreatment zone adjusted to have the temperatures shown in Table 2 tosubject the films to the heat-treatments for the periods shown in Table2, with the films being kept in the drawn and held state. Thereafter,the films were cooled to a room temperature to obtain biaxially orientedfilms of a polylactic acid resin with a thickness of 25 μm. InComparative Example 2, a film was cooled to a room temperature withoutundergoing the heat treatment, to obtain a biaxially oriented film of apolylactic acid resin with a thickness of 25 μn. Evaluation results ofthe physical properties of the films obtained are shown in table 2.TABLE 2-1 Ex- Ex- Ex- Example Example ample 1 ample 2 ample 3 Example 4Example 5 Example 6 Example 7 Example 8 Example 9 10 11 BlendCrystalline 100 100 90 80 80 70 ratio polylactic (wt parts) acid (a)Crystalline 100 80 60 50 40 polylactic acid (b) Amorphous 0 0 0 10 20 2020 30 40 50 60 polylactic acid (c) Drawing ratio in the MD 2.5 3.0 3.02.5 3.0 3.5 2.5 2.5 3 3 2.5 direction (times) Drawing ratio in the TD 65.1 4.5 6.2 6.2 5.1 4.5 6.3 5.5 6.0 6.0 direction (times) Heat Temp. (°C.) 140 140 140 140 140 140 130 130 140 140 130 treatment Time (sec) 3 33 3 3 3 3 3 3 3 5 conditions Storage MD 360 370 490 210 160 220 150 11090 60 30 modulus direction E′ (MPa) TD 160 150 250 95 80 100 70 60 70 4030 direction Heat MD 0.5 0.5 0.5 0.7 0.7 1.0 1.0 2.5 2.5 4.5 8.0shrinkage direction at 80° C. TD 1.0 0.5 0.3 1.2 1.5 1.5 2.5 3.0 3.5 4.58.5 for 10 direction sec (%) Heat of crystal fusion 31 31 40 28 25 35 2524 26 25 20 ΔHm (J/g) Haze (%) 0.9 0.8 1 0.7 0.5 0.9 0.5 0.5 0.7 0.6 0.5Tear TD 55 80 190 70 80 140 180 79 81 80 130 strength direction (mN)Coating adaptability ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ High speed cutting ⊚ ⊚ ◯ ⊚ ⊚◯-⊚ ◯ ⊚ ⊚ ⊚ ◯- ⊚ property (envelope window affixing adaptability)

TABLE 2-2 Comp. Comp. Comp. Comp. Comp. Comp. Example 1 Example 2Example 3 Example 4 Example 5 Example 6 Blend Crystalline polylactic 100100 80 ratio acid (a) (wt parts) Crystalline polylactic 90 50 30 acid(b) Amorphous polylactic 0 0 10 20 50 70 acid (c) Drawing ratio in theMD 3.0 3.0 2.5 3.0 2.5 2.5 direction (times) Drawing ratio in the TD 3.06.0 3.5 2.5 3.8 6.0 direction (times) Heat Temp. (° C.) 140 Nil 140 140130 130 treatment Time (sec) 3 3 3 5 5 conditions Storage MD direction370 70 290 150 40 4 modulus E′ TD direction 160 140 130 70 25 7 (MPa)Heat MD direction 0.5 >20 0.8 1.0 5.0 13 shrinkage TD direction 0.5 >200.5 1.2 6.0 15 at 80° C. for 10 sec (%) Heat of crystal fusion 31 30 3825 25 16 ΔHm (J/g) Haze (%) 0.8 0.7 0.9 0.5 0.6 0.5 Tear TD direction280 50 250 300 220 170 strength (mN) Coating adaptability ⊚ X ⊚ ⊚ ◯ XHigh speed cutting property X ⊚ X X X ◯ (envelope window affixingadaptability)

As shown in Table 2, the biaxially oriented films of a polylactic acidresin obtained in Examples 1 to 11 are found to be superior in thecoating adaptability, high-speed cutting property and envelope windowaffixing adaptability.

Examples 12 to 16

In Examples 12 to 16, pellets of crystalline polylactic acid (a) or (b)and amorphous polylactic acid (c) shown in Table 1 were dry blended tothe compositions shown in Table 3, followed by melt blending using aco-rotating twin screw extruder, extruding molten resins at a resintemperature of 200° C. through a T-die, quenching by a casting roll keptat a temperature of 35° C. to obtain substantially amorphous sheets.Then the sheets obtained were heated at 75° C., roll-drawn in the MDdirection at the drawing ratios shown in Table 3, and then tenter-drawnin the TD direction at 85° C. at the drawing ratios shown in Table 3.Then, these films were introduced into a heat treatment zone adjusted tohave the temperatures shown in Table 3 to subject the films to the heattreatment for the periods shown in Table 3, with the films being kept inthe drawn and held state. Thereafter, the films were cooled to a roomtemperature to obtain biaxially oriented films of the polylactic acidresins with a thickness of 20 μm. Evaluation results of physicalproperties of the films obtained are shown in Table 3 TABLE 3 Example 12Example 13 Example 14 Example 15 Example 16 Blend Crystalline 100 90 80ratio polylactic acid (a) (wt parts) Crystalline 100 70 polylactic acid(b) Amorphous 0 0 10 20 30 polylactic acid (c) Drawing ratio in the 2.52.5 2.3 2.6 2.5 MD direction (times) Drawing ratio in the 5.5 5.1 5.25.5 4.8 TD direction (times) Heat Temp. (° C.) 140 140 130 130 130treatment Time (sec) 3 3 3 3 3 conditions Storage MD 360 460 200 150 150modulus E′ direction (MPa) TD 150 260 100 80 80 direction Heat MD 0.50.5 0.7 1.0 1.2 shrinkage direction at 80° C. TD 1.0 0.5 1.3 1.7 1.8 for10 sec direction (%) Heat of crystal fusion 31 41 28 25 32 ΔHm (J/g)Haze (%) 0.6 0.7 0.5 0.4 0.5 Tear TD 40 90 55 60 110 strength direction(mN) Coating adaptability ⊚ ⊚ ⊚ ⊚ ⊚ High-speed cutting ◯-⊚ ◯-⊚ ◯-⊚ ◯-⊚ ◯property (envelope window affixing adaptability)

As shown in Table 3, the biaxially oriented films of a polylactic acidresin obtained in Examples 12 to 16 are found to be superior in thecoating adaptability, high-speed cutting property and envelope windowaffixing adaptability.

The polylactic acid resins used in the following Examples 17 to 27 andComparative Examples 7 to 10 were crystalline polylactic acid (a) andamorphous polylactic acid (b) having the weight average molecularweights and optical purities shown in Table 4, and obtained bypolymerization under the controls of catalyst amount, polymerizationconditions, monomer composition or the like in accordance with Examples1B to 7B in JP-A-4-504731. However, the compositions of polylactic acidresins in the present invention should not be limited thereto. TABLE 4Content of Weight D-lactic Optical average Mw acid purity Tg TmCrystalline 290,000 4.5% 91% 59° C. 153° C. polylactic acid (a)Amorphous 300,000 12.5% 75% 53° C. None polylactic acid (b)

Examples 17 to 25, 27 and Comparative Examples 7 to 9

In Examples 17 to 25, 27 and Comparative Examples 7 to 9, pellets ofcrystalline polylactic acid (a) and amorphous polylactic acid (b) shownin Table 4 were dry blended to the composition shown in Table 5,followed by melt blending using co-rotating twin screw extruder,extruding the molten resins at a resin temperature of 200° C. through aT-die, quenching by a casting roll kept at a temperature of 35° C. toobtain substantially amorphous sheets. Then the sheets obtained wereheated at 75° C. and roll-drawn at a drawing ratio of 2.5 in the MDdirection, and then at 80° C. tenter-drawn at a drawing ratio of 6 inthe TD direction. In Examples 17 to 25, 27 and Comparative Examples 7,the films were then introduced into a heat treatment zone adjusted tohave the temperatures shown in Table 3 to subject the films to the heattreatment for the periods shown in Table 5, with the films being kept inthe drawn and held state. Thereafter, the films were cooled to a roomtemperature to obtain biaxially oriented films of the polylactic acidresin with a thickness of 25 μm. In Comparative Example 8 and 9, thefilms were cooled to a room temperature without undergoing theheat-treatment to obtain biaxially oriented films of the polylactic acidresins with a thickness of 25 μm. Evaluation results of physicalproperties of the films obtained are shown in table 5.

Example 26

In Example 26, a film was oriented in the same manner as in Examples 17to 25 and 27, except that polylactic acid (a) and (b) shown in Table 4were used and that the drawning ratios were 3 in the MD direction and5.5 in the TD direction as shown in Table 5. The film was then subjectedto the heat treatment under the conditions shown in Table 5 and cooledto a room temperature to obtain a biaxially oriented film of apolylactic acid resin with a final thickness of 25 μm by adjusting thedegree of die-lip opening. Evaluation results of the physical propertiesof the films obtained are shown in table 5.

Comparative Example 10

In Comparative Example 10, a film having the same composition as inExample 27 was drawn in the same manner as in Example 27, except thatthe film was drawn at ratios of 2.5 in the MD direction and 3 in the TDdirection as shown in Table 5. The film was subjected to the heattreatment under the conditions shown in Table 5 and cooled to a roomtemperature to obtain a biaxially oriented film of a polylactic acidresin with a final thickness of 25 μm by adjusting the degree of die-lipopening. Evaluation results of the physical properties of the filmobtained are shown in table 5. TABLE 5-1 Ex- Ex- Ex- Ex- Ex- Ex- Ex- Ex-Ex- Ex- Ex- ample ample ample ample ample ample ample ample ample ampleample 17 18 19 20 21 22 23 24 25 26 27 Blend Crystalline 80 75 70 60 5040 95 90 85 80 100 ratio polylactic (wt parts) acid (a) Amorphous 20 2530 40 50 60 5 10 15 20 0 polylactic acid (b) Drawing ratio in the MD 2.52.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 3.0 2.5 direction (times) Drawing ratioin the TD 6 6 6 6 6 6 6 6 6 5.5 6 direction (times) Heat Temp. (° C.)130 130 130 130 120 120 140 140 140 140 140 treatment Time (sec) 3 3 3 35 5 3 3 3 3 3 conditions Storage MD direction 150 130 110 70 40 20 250210 180 170 350 modulus E′ TD direction 70 65 60 50 25 20 110 95 80 65150 (MPa) Heat MD direction 1.0 1.8 2.5 3.8 4.5 8.0 0.5 0.7 0.8 1.0 0.5shrinkage TD direction 2.5 2.5 3.0 4.5 4.5 8.5 1.0 1.2 1.5 1.2 1.0 at80° C. for 10 sec (%) Heat of crystal fusion 25 24 23 21 18 16 30 28 2625 32 ΔHm (J/g) Haze (%) 0.5 0.4 0.4 0.3 0.3 0.4 0.8 0.7 0.6 0.5 0.9Tear strength TD 80 80 79 81 75 130 75 79 79 90 79 (mN) directionTensile MD 95 90 90 80 75 69 100 100 100 110 110 strength (MPa)direction TD 175 170 160 140 125 120 180 180 180 180 190 directionTensile MD 190 190 190 200 210 220 190 190 190 180 190 elongation (%)direction TD 90 90 90 100 100 110 80 80 80 100 80 direction Coatingadaptability ⊚ ⊚ ⊚ ◯ ◯ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ Heat seal property ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚⊚ X (hot tack strength/inch 9 10 11 13 15 17 5 7 8 9 2 width High speedcutting ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ property (envelope window affixingadaptability)

TABLE 5-2 Comp. Example 7 Comp. Example 8 Comp. Example 9 Comp. Example10 Blend Crystalline 30 100 80 100 ratio polylactic (wt parts) acid (a)Amorphous 70 0 20 0 polylactic acid (b) Drawing ratio in the MDdirection (times) 2.5 2.5 2.5 2.5 Drawing ratio in the TD direction(times) 6 6 6 3 Heat treatment Temp. (° C.) 120 No treatment Notreatment 140 conditions Time (sec) 5 3 Storage modulus E′ MD direction2 55 40 310 (MPa) TD direction 5 250 160 160 Heat shrinkage at 80° C. MDdirection >20 >20 >20 0.5 for 10 sec (%) TD direction 11 >20 >20 0.8Heat of crystal fusion ΔHm (J/g) 12 31 25 31 Haze (%) 0.4 0.7 0.5 0.8Tear strength (mN) TD direction 140 60 65 250 Tensile strength (MPa) MDdirection 65 140 100 100 TD direction 100 250 200 120 Tensile elongation(%) MD direction 250 200 210 150 TD direction 130 80 80 160 Coatingadaptability X X X ⊚ Heat seal property ⊚ X ⊚ X (hot tack strength/inchwidth 19 2 9 2 High speed cutting property ◯ ⊚ ⊚ X (envelope windowaffixing adaptability)

As shown in Table 5, the biaxially oriented films of the polylactic acidresins obtained in Examples 17 to 27 are found to have tensile strengthsof not lower than 60 MPa, that is, the mechanical strength sufficientfor films to be used as bags. At the same time, these biaxially orientedfilms of a polylactic acid resin are found to be superior in the coatingadaptability and high-speed cutting property.

INDUSTRIAL APPLICABILITY

The biaxially oriented film of a polylactic acid resin of the presentinvention is biodegradable as it mainly comprises a polylactic acidresin and has the superior coating adaptability for an antistatic agent,a lubricant, an antiblocking agent or the like, and the superiorhigh-speed cutting property. Therefore, the present invention canprovide a biaxially oriented film of a polylactic acid resin suitablefor bags, packaging film and plastering film for various windows,particularly those for outlook window envelopes.

1. A biaxially oriented film of a polylactic acid resin comprising notless than 50% by weight of a polylactic acid resin, which has a storagemodulus E′ at 80° C., in at least one of a longitudinal direction (MDdirection) and a width direction (TD direction) of the film, of from 10MPa to 3,000 MPa, as determined by a test method for temperaturedependency of dynamic viscoelasticity in accordance with JIS K7198(method A); a heat shrinkage of not higher than 10%, upon heating at 80°c for 10 seconds; and a tear strength in the width direction (tddirection) of 10 to 200 mN, as determined in accordance with JIS K7128(method B).
 2. The biaxially oriented film of a polylactic acid resin inaccordance with claim 1, wherein the storage modulus E′ at 80° C., in atleast one of the longitudeinal direction (MD direction) and the widthdirection (TD direction) of the film, is within the range from 50 MPa to1,000 MPa, as determined by the test method for temperature dependencyof dynamic viscoelasticity.
 3. The biaxially oriented film of apolylactic acid resin in accordance with claim 1 or 2, wherein thestorage modulus E′ at 80° C., in at least one of the longitudeinaldirection (MD direction) and the width direction (TD direction) of thefilm, is within the range from 10 MPa to 300 MPa, as determined by thetest method for temperature dependency of dynamic viscoelasticity; and aheat of fusion, ΔHm, at the crystal melting peak, present at atemperature not lower than 100° C., is in the range from 15 to 30 J/g,as determined by a differential scanning calorimeter (DSC) with atemperature being increased from 0° C. to 200° C. in accordance with JISK7122.
 4. The biaxially oriented film of a polylactic acid resin inaccordance with claim 1 or 2, wherein the polylactic acid resin is amixture comprising 95 to 60 parts by weight of crystalline polylacticacid with an optical purity of not lower than 85% and 5 to 40 parts byweight of amorphous polylactic acid with an optical purity of not higherthan 80%.
 5. A window for an outlook window envelope, comprising thebiaxially oriented film of a polylactic acid resin in accordance withclaim 1 or
 2. 6. The biaxially oriented film of a polylactic acid resinin accordance with claim 1 or 2, wherein said film is drawn at a ratioof not less than 4 in the width direction (TD direction) of the film,and is subjected to a heat treatment at a temperature not lower than theglass transition temperature (Tg) thereof and not higher than themelting point (Tm) thereof.
 7. A method for producing a biaxiallyoriented film of a polylactic acid resin, comprising: drawing a filmcomprising a resin containing not less than 50% by weight of a mixturecomprising 95 to 60 parts by weight of crystalline polylactic acid withan optical purity of not lower than 85% and 5 to 40 parts by weight ofamorphous polylactic acid with an optical purity of not higher than 80%,at a ratio of not less than 4 in the width direction (TD direction) thefilm; and subsequenty subjecting the film to a heat treatment at atemperature not lower than the glass transition temperature (Tg) thereofand not higher than the melting point (Tm) thereof.